1. Field of Invention
The present invention relates to the technical field of hydraulic pumps equipped on working machinery such as hydraulic shovels.
2. Description of Related Art
Generally, some working machinery such as hydraulic shovels, for example, are equipped with a variable displacement hydraulic pump driven by the engine power, and are designed to supply pressurized oil delivered from the hydraulic pump to a plurality of hydraulic actuators through directional control valves whose opening degrees varies depending on stroke shifts of operating units. To supply the pressurized oil to the plurality of hydraulic actuators, which are operated in a combined manner, at flow rates neither under nor over proper values, the torque input or power input to the variable displacement pump--hereinafter referred to as a pump absorbing torque (or absorbing horsepower) is required to be controlled with respect to an engine torque (or engine horsepower) while keeping a good balance so that an actual revolutions per unit time--hereinafter referred to as revolution number of the engine follows a target revolution number thereof.
In view of such an requirement, a shown in FIG. 10, it has been hitherto proposed to control a torque control pressure Ps supplied to pump regulators 12, 13 by using a controller 30.
Specifically, in FIG. 10, the controller 30 receives detection signals from a revolution number sensor 22 for detecting a revolution number of the engine 11 and a pressure switch 31 for determining whether hydraulic pumps 9, 10 are delivering pressurized oil. Then, the controller 30 outputs a control signal to a solenoid proportional reducing valve 14 for controlling a total absorbing torque (or horsepower) of the hydraulic pumps so that the engine revolution number follows a target revolution number. The control signal is subject to electro-hydraulic conversion by the solenoid proportional reducing valve 14, and a resulting torque control pressure Ps is supplied to regulators 12, 13.
In the conventional torque (horsepower) control, however, detection signals necessary for calculating oil amounts (flow rates) delivered from the hydraulic pumps (e.g., detection signals indicating stroke shifts of operating units) are not input to the controller, and there is a difficulty in accurately estimating the absorbing torque required by the hydraulic pumps. This has raised a problem that a balance between the engine output and the pump absorbing torque is lost just before start and after end of manipulation of the operating units or when the operating units are manipulated slightly, and a deviation of the actual revolution number from the target revolution number of the engine is so increased as to deteriorate operability. That problem is to be overcome by the present invention.
Also, an adjustment process of conventional controllers requires tuning for each of different models of working machinery even if they belong to a similar type of working machinery. In other words, the adjustment process has been troublesome because of the necessity of executing specific parts of the control program separately for each model.
Further, there is a difference between specific units of working machinery that are even the same model. In addition, working environment depends on the ambient conditions at sites (e.g., a cold district or a warm district), and engine fuel may be changed depending on users. Changes in various conditions such as the differences between specific units of working machinery and working environment have raised another problem to be overcome that the tuning made before shipping of working machinery is not adaptable practically and a deviation of the actual revolution number from the target revolution number of the engine is increased to an unallowable level.